The productivity of a vertical outdoor photobioreactor was quantitatively assessed and compared to a horizontal reactor. Daily light cycles in southern Spain were simulated and applied to grow the microalgae Chlorella sorokiniana in a flat panel photobioreactor. The maximal irradiance around noon differs from 400 μmol photons m(-2) s(-1) in the vertical position to 1800 μmol photons m(-2) s(-1) in the horizontal position. The highest volumetric productivity was achieved in the simulated horizontal position, 4 g kg culture(-1) d(-1). The highest photosynthetic efficiency was found for the vertical simulation, 1.3g of biomass produced per mol of PAR photons supplied, which compares favorably to the horizontal position (0.85 g mol(-1)) and to the theoretical maximal yield (1.8 g mol(-1)). These results prove that productivity per unit of ground area could be greatly enhanced by placing the photobioreactors vertically.